Sampling for Sulphuric Acid Mist

Tomorrow I’m travelling over to Leeds to attend a seminar organised by the Chemical Industries Association seminar  on Controlling and Measuring Occupational Exposure to sulphuric acid mist. I’ll be making a brief presentation on sampling methodologies. I’ve uploaded a copy of the presentation to my Slideshare site, and it can be viewed below.

Exposure to sulphuric acid mist can occur in a number of industries including during sulphuric acid manufacture, loading and unloading tankers, transfer between storage vessels, charging lead acid batteries, pickling and plating operations and other processes.


A new Workplace Exposure Limit (WEL) for sulphuric acid mist came into force in December last year when the UK implemented the latest batch of European indicative occupational exposure limit values (IOELVs). The most notable feature about this WEL, besides having a low value of 0.05 mg/m3, the limit refers to the thoracic fraction. This is the first thoracic limit that has been set, all other particulate WELs being for either the inhalable or respirable fractions.


It is questionable whether a thoracic limit is appropriate for sulphuric acid. The main concern, besides upper respiratory tract irritation, is the potential for tumour formation in the respiratory tract, believed to be a consequence of sustained tissue inflammation and repair processes, with the larynx the main site of concern. As the thoracic fraction excludes particles that don’t penetrate beyond the larynx, it doesn’t really seem appropriate for a limit to be set that excludes the main particles of concern. The original recommendation from SCOEL (the European Union’s Scientific Committee on Occupational Exposure Limits) was for a limit based on the inhalable fraction, but by the time it reached the EU Council it had become a thoracic limit and that’s what ended up in the EU directive and, ultimately, as a UK WEL.


The difficulty the thoracic limit presents is that, currently, we don’t have a validated method for measuring the thoracic fraction of the mist. There are some options, but work needs to be done to determine which method is most suitable and to properly validate it. Given the current political climate with major cutbacks in expenditure by the Health and Safety Executive, I think it’s highly unlikely that they’ll fund this.

The easiest approach is to carry out sampling for the inhalable fraction. If the results are below 0.05 mg/m3, then as the thoracic is a sub fraction of the inhalable convention, then they would demonstrate compliance with the WEL. If the results are substantially greater than 0.05 mg/m3, then there’s a good chance that the WEL will have been exceeded and additional controls need to be introduced. However it would be difficult to interpret results that are only just higher than 0.05 mg/m3.

Sampling for sulphuric acid mist version for slideshare

View more presentations from Mike Slater
  1. European Commission, Employment, Social Affairs and Inclusion, January 2007 Recommendation from the Scientific Committee on Occupational Exposure Limits for sulphuric acid SCOEL/SUM/105
  2. Health and Safety Executive, 2011, EH40/2005 Workplace Exposure Limits, Second Edition

Some implications of exposure variability

In our last post we saw how inhalation exposures to hazardous in the workplace are highly variable. The spread of results from an air sampling survey is usually quite wide and will usually conform to a skewed log-normal distribution.


This means that although there is a large spread of results the majority of exposures are at the lower end of the range with a relatively small number of high exposures. Nevertheless these higher results are valid and cannot be discarded.

This has a number of important implications for the design of air sampling surveys and the interpretation of the results. In particular for the number of samples that need to be taken to allow a clear picture of  the range of exposures to be obtained.

If only one or two samples are taken for a particular process or task then it is most likely that they will be from the lower end of the range where the bulk of the results are clustered. It is less likely that a result from the higher end of the range, which could be 3 or more times greater,  will be obtained. Care has to be taken, then when interpreting the results to decide whether exposure is adequately controlled. If the results from the small data set are close to the relevant occupational exposure limit, then it is likely that some workers on some occasions are likely to have exposures that will exceed the limit. Even if the results are substantially below the limit, this does not necessarily guarantee that exposure is adequately controlled.

One way to avoid this problem is to take enough samples so that a reasonable estimate of the range of exposures can be obtained. This is easier than it sounds. How many samples are enough? Unfortunately, guidance is sketchy.

The British Health and Safety Executive, in Appendix 2 of their guidance document HS(G) 173 “Monitoring strategies for toxic substances”, gives the following advice on the number of samples required is given:

  • for worst case sampling, 1 in 5 workers should be monitored, unless a smaller number can be justified;
  • for representative measurements, if there are fewer than 10 employees, only 5 need to be included, but although caution is advised for "more complex situations involving more then ten employees", no further advice is given on how to proceed.

However, many occupational hygienists would consider that even in simple cases, considerably more than 1 in 5 workers need to be included.

The American Industrial Hygiene Association, in their A Strategy for Assessing and Managing Occupational Exposures, suggest that 6 to 10 measurements are normally enough to obtain a reasonable estimate of worker exposures from each group.  It will usually be practicable to collect this number of samples during a one day survey. However, there are many situations where only a small number of workers, fewer than this, carry out a given task. It may require repeated sampling on  several days to obtain 6 samples

It is worth bearing in mind that, a better picture of exposure will be obtained if a larger proportion of workers are covered by the survey.  A little common sense needs to be used, balancing the value of the results against the cost of the survey. Sampling is expensive and there is always pressure from clients to keep costs down.

The  British Occupational Hygiene Society and the Nederlandse Vereniging van Arbeidsdeskundigen, have recently published a draft document on Testing Compliance with OELs for Airborne Substances (available here) which addresses these issues. It’s not an easy read but there will be a workshop on the proposals during the first day of the forthcoming BOHS Autumn Scientific Conference taking place in Leeds on 14th and 15th  November. I’m sure there will be other opportunities to discuss it in the future. I’ll be returning to it in a future post.

Variability in exposure measurements

Whenever an occupational hygienist carries out an air sampling survey, either for a group of workers carrying out the same tasks in a similar way, or for the same individual worker carrying out the same job on different days, it’s inevitable that a range of results will be obtained.  However, although most people would probably accept that it would be unreasonable to expect all the results to be the same, in my experience many people, including health and safety professionals don’t fully appreciate just how much variation there is in worker exposures.

To illustrate this, let’s look at an example.


The following results were obtained from a survey for sulphuric acid mist during anodising of metal components. This is an electrolytic process where sulphuric acid to create an protective or decorative oxide layer on the surface of  the metal. The process is called “anodising” because the component forms the anode electrode of an electrical circuit.


As you can see, there is quite a wide spread, ranging from 0.04 mg/m3 to 0.20 mg/m3 . The highest result is 5 times higher than the lowest. Many people find this surprising, but it’s not untypical for sampling results. You might also have noticed that the results are not evenly distributed across the range. This can be seen more easily if we plot them on a histogram


Here’s another set of results from the same facility from a survey carried out at a later date


And plotted as a frequency histogram:


Again we can see a similar pattern but with a larger spread – the highest result is seventeen times higher than the lowest.

The pattern evident in these two examples is not untypical for sampling results. What we can see is that

  1. there is a relatively wide spread of results
  2. the majority are at the lower end of the range
  3. there are a small number of results that considerably higher than the majority

Statistically, we typically find that the sample results for a group of “similarly exposed” workers conform to a log-normal distribution. This is a skewed distribution with the majority of results bunched together,  but with a small, though significant, number of very high results.


There is often a tendency to consider these low frequency high results as untypical and the temptation is to exclude them, particularly if they exceed the relevant exposure limit. Sometimes that may be valid – if a good reason can be found for rejecting them. But in many cases the high results are simply part of the natural variation in exposures and can’t be ignored.

There are other important implications too. Particularly for interpreting the results from surveys where only a few samples were taken. It is more likely that the results will be from the lower end of the distribution as the higher results have a low frequency.

Understanding variation is important for occupational hygienists as it has major implications for how we design our surveys  and interpret the results.

Measuring hazardous substances

To me, the main aim of occupational hygiene is controlling exposure to hazardous agents, including hazardous substances, to do that its important to understand the degree of risk. That’s the only way we can ensure we’re directing our effort, energy and money at the real problems. With hazardous substances, measurement of their airborne concentration is often needed to help us to understand the risks, and so is a very important part of the science and practice of occupational hygiene.

This week we’re running the BOHS Module M102 “Measurement of Hazardous Substances”. in Chester, While reviewing the materials in preparation for the course, it occurred to me that the methods we use to measure hazardous substances haven’t really changed since I started work just over thirty years ago.  We still  capture particulates on a filter held in a “sampling head” which determines the particle size distribution captures, used in conjunction with a small sampling pump. Adsorption tubes are still the main approach used to assess exposure to organic vapours.


There have been some developments. The IOM sampler is now the preferred sampling head for inhalable particulates, rather than the old “7 hole head”, but this was a redesign and development of an exiting technique, rather than an entirely new approach. The use of size selective foams in conjunction with the IOM sampler has made it easier to sample for the respirable fraction, although there are some doubts as to whether this method is valid for all types of dust.


Equipment manufacturers have made some improvements to sampling pumps. They are generally lighter, more reliable with a more stable flow changes rather than radical new developments.  In some cases manufacturers have added extra “bells and whistles” to their pumps, but these “enhancements”, which tend to increase the cost of the devices, are rarely necessary.

The standard methods work, so there hasn’t been any need for major changes.  The old saying, “if it isn’t broke, don’t fix it” comes to mind. They are relatively easy to use, usually reliable and, an important consideration, they are relatively inexpensive. These are important factors, as, due to the large range of variability in occupational exposures, it’s desirable to collect as many samples as practicable to achieve a reasonably accurate assessment of exposure and the associated risk.

Having said that, sometimes something new comes along which requires us to re-evaluate how we do things. A recent example of this is the challenges presented by the measurement and assessment of nanoparticles. Although sub micron particles are not new – there are natural sources (e.g. forest fires, wood stoves, volcanoes) and are present in  traffic exhaust fume, welding fume, plastic fume and materials such as carbon black – the development and manufacture of “engineered” nanoparticles has increased dramatically in recent years. Research suggests that the toxicity of these particles is related to their surface area rather than their mass concentration, so the traditional methods we use for measuring dust exposures, which evaluate the latter, are unlikely to be appropriate. Although some methods are available for measuring nanoparticles, further work is needed to develop a cost effective method that can be used in the workplace.